The present invention relates to an apparatus for detecting changes in concentrations of components of a fluid mixture by relating changes in an electrical or mechanical property of a sensing means to such changes in concentrations. In particular, this invention relates to such an apparatus based on a change in a resistance of a sensing means made of an activated carbon material as the concentrations change.
Due to the capability of activated carbon to remove a wide variety of chemical compounds from fluid mixtures, it has been widely used to purify such mixtures. The removal is effected by physical adsorption of these contaminants in the micropores of the activated carbon which are developed during the manufacturing process. The activated carbon is said to be spent when its micropores are filled with the contaminants and it no longer has any capacity for further adsorption. When the activated carbon bed is spent, the concentration of a contaminant in the treated fluid stream increases rapidly. This event is referred to as the breakthrough of the go contaminant. To ensure a properly treated stream, the activated carbon bed must be replaced at some time prior to the first breakthrough of the contaminants. Since the concentrations and the types of contaminants may vary during the time during which the carbon bed is on stream, the breakthrough point cannot be precisely predicted and, therefore, the quality of the treated fluid stream must be monitored by frequent periodic testing, which sometimes can be costly. Therefore, there is a need for a device that can warn the user when the carbon bed is nearly spent without the requirement for periodic testing of the treated fluid. Such a device preferably stays on stream continuously and detects an increase in the contaminant level near the outlet of the carbon bed to provide an efficient use thereof. Such a device is also useful in purification systems employing media other than activated carbon.
Many sensors have been proposed in the art, but they have been directed to detecting only hydrocarbon contaminants and, thus, limited in their applications. U.S. Pat. Nos. 5,079,944 and 5,150,603 disclosed hydrocarbon vapor sensors and systems for detecting leaks from underground storage tanks. The heart of the sensors of these patents was a sensing element made of a conductive polymer, the resistance of which changed when hydrocarbons absorbed into and swelled the polymer matrix. In one embodiment, the conductive polymer comprised a polymeric tape carrying conductive carbon particles. In another embodiment, the conductive polymer was an elastomer or a silicone rubber filled with silver-coated glass spheres or metallic silver flakes. The presence of hydrocarbon vapor was ascertained when the resistance of the sensing element increased rapidly. However, this sensor would not be able to detect any contaminant that does not absorb readily into and swell the polymer matrix.
Moyer et al. disclosed the use of an end-of-service-life indicator for organic vapor cartridge respirators (E. S. Moyer et al., xe2x80x9cA Preliminary Evaluation of an Active End-of-Service-Life Indicator for Organic Vapor Cartridge Respirators,xe2x80x9d Am. Ind. Hyg. Asssoc. J., Vol. 54, No. 8, pp. 417-425 (1993)). The sensor of this study comprised a mixture of silicone rubber and carbon particles deposited on a substrate. The detection of organic vapor also relied on the swelling of the silicone matrix, resulting in an increase in the resistance of the silicone/carbon conducting film. Thus, a contaminant that does not swell the silicone would not be detected because the resistance of the film would not change.
Marchand reported the construction of an organic vapor sensor which comprised a small bed of activated carbon cloth (E. G. Marchand, Ph.D. Thesis, Michigan Technological University, 1996). Experiments under flow conditions were done only with trichloroethylene in dry air. The electrical resistance of the carbon cloth began to increase sharply when the challenge gas is admitted into the carbon cloth sensor long before the breakthrough occurred. There was no suggestion as to how a sensor may be configured to indicate reliably a breakthrough by directly relating a change in this electrical property to the breakthrough point.
Therefore, it is an object of the present invention to provide an apparatus for detecting changes in concentrations of a range of minor components of a fluid mixture. It is a further object of the present invention to provide a detection apparatus that relates the change in electrical resistance, or mechanical properties, of a carbon-based sensing element to the change in concentrations of the minor components. It is still another object of the present invention to provide an apparatus for detecting the breakthrough of minor components of a fluid mixture from an activated carbon bed used for the purification thereof.
The present invention provides an apparatus for detecting changes in concentrations, or amounts of minor components, of a fluid mixture that measures the changes in electrical resistance or mechanical properties of a sensing element when the sensing element is exposed to the fluid mixture. Minor components of a fluid mixture, as are referred to herein, are those having weight-based or molar concentrations less than about 50 percent. Such an apparatus will hereinafter also be referred to as a detector. The present invention measures changes in other electrical or mechanical properties of the sensing element, such as electrical capacitance, mechanical strength or dimensional change, when such changes occur in relation to changes in concentrations or amounts of the minor components of the fluid mixture. When the measured electrical or mechanical property of the sensing element changes, the detector indicates a change in a concentration. The means for detecting the measurable property varies depending upon the property and includes, for example, electrical detectors, Wheatstone bridge related technologies or stress or strain gauges. The present invention further includes, in another embodiment, a sensing element for use in detecting changes in concentrations minor components of a fluid mixture, said element comprising an activated carbon cloth having a connecting means for measuring said changes.
In general, the detector of the present invention comprises at least first and second sensing elements that are carbon-based, or are of other suitable materials, such as metal, and have substantially the same composition, electrical resistance and dimensions. The electrical resistances of the two sensing elements preferably differ by less than 10 percent; more preferably, less than 5 percent. One particularly suitable carbon for the manufacture of the sensing elements is activated carbon fiber or activated carbon cloth (xe2x80x9cACCxe2x80x9d) which is manufactured in thermal processes from raw materials made of woven or non-woven natural, man-made, or synthetic fibers and which possesses a high surface area for adsorption of a wide variety of compounds. ACC typically has a low electrical resistance, and this resistance changes substantially when the ACC adsorbs even a small amount of contaminants. This property is used advantageously in the detector of the present invention to detect contaminants in fluid streams.
Alternatively, the sensing elements may be made of an electrically conducting sheet comprising a conductively effective amount of activated carbon particles immobilized in a polymeric fiber mixture. The present invention may be used in conjunction with a system, such as a purification system. When used with a purification system, the first sensing element is exposed to the fluid mixture, the change in the concentration of one or more minor components is detected. Preferably, the first sensing element is positioned at a location where the fluid mixture initially has a first substantially stable composition. The second sensing element serves as a reference and remains exposed to the same fluid mixture having the first substantially stable composition. In many situations in which the fluid mixture is being purified by the removal of the minor components, the fluid mixture having the first substantially stable composition is that substantially devoid of the minor components. As the concentration of a minor component in this mixture increases, the changes in such concentration are detected. Initially, the electrical resistances or mechanical properties of the first and second sensing elements are substantially equal when exposed to the same fluid mixture having the first substantially stable composition. When the concentration of a minor component of the fluid mixture begins to change, measurable electrical or mechanical properties of the first sensing element also begin to change in comparison to that of the second sensing element when exposed to the fluid mixture near the outlet of the purification system. The present invention will detect that a minor component of the fluid mixture is about to breakthrough and a warning signal may be given.
In a preferred embodiment of the present invention, the detector comprises two sensing elements made of ACC which are spaced apart in an elongated enclosure having an inlet and an outlet. An adsorbent capable of removing the minor components of the fluid mixture is disposed in the elongated enclosure between the sensing elements. Examples of such an adsorbent are activated carbon that have the form, for example, of granules, pellets, or spheres; natural or synthetic zeolite; silica gel; activated alumina; and any of these adsorbents modified to enhance their adsorption capacities. Electrically conducting leads are provided to measure the electrical resistance of each of the carbon-cloth sensing elements. The fluid mixture, having a first substantially stable composition, enters the detector and contacts, in sequence, the first sensing element, the adsorbent portion, and the second sensing element. This fluid stream is preferably a split stream of the fluid mixture flowing through the enclosure and is taken from a point near the outlet of the purification system. Initially, the compositions of the fluid contacting the first and second sensing elements are substantially the same and both sensing elements exhibit substantially the same electrical or mechanical properties. The properties are used to establish a base line.
When used with a purification system and the capacity of the purification system has been exhausted, the concentrations of the minor components of the fluid mixture begin to increase. The adsorption of the minor components in the ACC material of the first sensing element results in a change in the electrical resistance thereof. The change is positive or negative depending on the types of the minor components and the fluid mixture. Due to the adsorption of the minor components in the adsorbent portion between the first and second sensing elements, the resistance of the second sensing element remains at the base line value for a short time thereafter. Thus, a differential in the resistances indicates that a breakthrough of the minor components from the purification system is about to occur. A means for detecting the differential is provided and can be designed to notify an operator of the system so that appropriate actions may be taken to ensure a continued quality of the treated fluid stream. Several similar detectors of the present invention may be used in conjunction with a purification system to indicate the degree of exhaustion of the capacity of the purification system. When used in this manner, the detectors are arranged at intervals along the system.
In another embodiment of the present invention, each carbon-based sensing element is housed in a separate enclosure. The first sensing element is exposed to a stream of the fluid mixture, the change in the composition of which is desired to be detected. This first sensing element is located before the outlet of the purification system to detect the minor components before they breakthrough. The second sensing element serving as the reference is always exposed to a split stream of the treated fluid. At the beginning of the purification operation, the compositions of the fluid streams flowing through the sensing elements are substantially the same. Thus, the differential in the resistances of the sensing elements is near zero. As the first minor component of the fluid mixture reaches the location of the first sensing element, its resistance begins to change rapidly while that of the reference sensing element remains substantially constant. This change signals that the first minor component is about to breakthrough from the fluid treatment system.